We propose a randomized controlled trial to evaluate how well a weight loss intervention based primarily on text messages sent to/from mobile phones, mDIET, promotes weight loss and weight loss maintenance in overweight and obese men and women age 21 through 60. This study is based on promising formative research and a pilot randomized controlled trial (RCT) recently concluded among 75 overweight/obese men and women (mean age: 49.9) with the support of an NCI R21. To our knowledge the mDIET pilot study was the first RCT to address overweight/obesity with an intervention that is based primarily on the use of text (SMS) messages. At 16 weeks we found a significant between-group difference in weight with mDIET users losing 3.16% more weight (2.88 kg) than controls and very high acceptability. SMS messaging is becoming increasingly common and can be viewed on essentially all mobile phone platforms. Thus, the population reach of behavioral interventions using this approach can be extensive. In this project we will expand and strengthen the expert system logic underlying mDIET, strengthen content targeted at energy intake behaviors that contribute to weight loss and develop additional content related to physical activity and sedentary behavior -- all changes aimed at enabling mDIET to promote and sustain a 5-10% weight loss at 12 months. We will conduct additional formative research among those whose first language is Spanish, and develop mDIET for both English and Spanish-language speaking men and women. Then, 309 overweight/obese (BMI 25.1-39.9) women and men, age 21-60 will be randomized to one of three 12-month conditions: a) an SMS only version of mDIET;b) An SMS+Phone version of mDIET that includes regular brief counseling calls from a trained weight loss specialist;or c) a usual care Standard Print-based weight intervention. We will enroll approximately 30% in the Spanish language preference group to assure our ability to perform meaningful exploratory analyses in this population. mDIET is based in behavioral theory and can support delivery of tailored messages to each user. The primary outcome will be the effect of mDIET on percent weight loss at 12 months. Secondary outcomes will include assessments at 6 and 12 months of mDIET effects on proportion of each group that lose at least 5% of initial body weight, behavioral, and psychosocial constructs, quality of life &depression, satisfaction with the intervention and process measures related to use of intervention elements. PUBLIC HEALTH RELEVANCE: This project explores whether a program based mainly on the use of automated text messages sent to and from a person's mobile phone can help overweight/obese people lose weight. Messages are sent a few times each day based upon expert system rules and promote improved behaviors important to weight loss and weight loss maintenance. Some messages request a reply encouraging continued engagement and interaction with the program. Because of the widespread use of text messages, if found efficacious the public health impact of this program could be great.